1. Field of the Invention
The present invention relates to multilayer circuit boards and folded flexible circuit boards.
2. Description of the Related Art
Primarily flexible circuits are used to solve problems with interconnection of rigid circuit boards. This new structure offers the advantages of increased reliability, weight and space savings, a reduction in mechanical connectors, and greater impedance control.
Four types of flexible circuits are prevalent today: single-side, double-sided, rigid-flex and multilayer. Single-sided circuits, consisting of a single layer of conductors on a base film, are most commonly used in interconnection applications. A double-sided flexible circuit has a single-etched conductive pattern on both sides of the base film. It is used when circuit density and layout cannot be routed on one layer, and in ground plane applications.
Rigid-flex circuitry traditionally consists of both rigid and flexible substrates selectively laminated together into a single cohesive construction, electrically interconnected with electroplated holes or xe2x80x98viaxe2x80x99. The rigidized area can be used to mechanically reinforce the circuit board in an area subject to increased abrasion or other forms of stress.
Multilayer flexible circuits consist of a sandwich formed by many layers of copper, between the dielectric substrates, and are used primarily in high-density applications. Conventional multilayer printed circuit boards consist of a number of thin boards sandwiched together between layers of epoxy resin-impregnated glass cloth. Connections between layers are achieved by means of plated via holes.
In the prior art, there have been some attempts to provide foldable flexible circuit boards.
For example, U.S. Pat. No. 4,928,206 by Porter et al. discloses a number of rigid circuit boards connected by a number of flexible circuit panels. Integrated circuit boards are mounted on the rigid printed circuit boards. The boards are foldable to reduce the entire unfolded layout in one direction, either width or length. Such a configuration is attached serially and folds along the flexible portions from a completely unfolded position to a most compact folded position.
Another flexible printed circuit board, by Takao et al. (U.S. Pat. No. 5,917,158), discloses a single circuit board having a main body and a folding portion as part of the single circuit board and wherein a set of copper foil lines provided on both the main body and the folding portion so that the folding portion overlaps the main body so that terminals at the ends of the copper foil lines make electrical contact with each other.
Furthermore, U.S. Pat. No. 5,398,163 by Sano discloses a flexible printed circuit board having at least one electrically conductive layer, and a fold retainer pattern which is electrically isolated from the printed circuit pattern. The fold retainer pattern maintains its folded shape once the flexible sheet is folded.
In the above devices, as well as in other prior art devices, there are a series of rigid circuits which in one way fold via flexible connections, to reduce the open layout of the circuitry in one direction. In the above patents, the folding circuit board serves to reduce the unfolded area of the circuit board in order to minimize the space taken up by the circuit board, or fit a series of rigid circuit boards by flexible connectors in order to conform to the desired space available. No consideration is given to the nature of the folding, or situations where there are theoretical limitations to the space available or very low current signals where one seeks to minimize inter-channel crosstalk or capacitance.
However, in certain electronic devices further reductions in compactness is required, specifically in circuit boards utilizing high density of conductor lines where space constraints limit the circuit board size. For example, in a Kinestatic Charge Detector (KCD, U.S Pat. No. 4,764,679 by McDaniel et al.), a strip-beam, multielectrode ionization chamber is used to produce a high-resolution digital radiographic image of a subject. The KCD utilizes a high-pressure rare gas as the uniform detection medium enclosed in a tubular chamber. X-rays after passing through the subject enter the chamber and ionize the gas forming charge pairs (ion-electrons). An externally induced precisely controlled electric field within the chamber is used to direct the positive ions at a constant velocity towards the collector electrodes or channels. The collector electrodes accumulate the current induced by the ions and the signal from these electrodes is read out by the interfacing electronics. In other words, the uniform detection medium contains spatial information that is read out as the ions approach the collectors.
Current circuit board manufacturing technology sets the minimum separation between conductor lines of a KCD at 0.013 cm for a conductor width of 0.013 cm for large circuit boards. (For a much smaller circuit board area, smaller conductor widths and separations on the order of a couple of hundred nanometers are possible.) According to the current technology, it would require, at least, 15 cm of circuit board width just to lay 576 conductors (signal channels for the KCD). These conductors need to be taken out of the chamber with the help of vias that need a hole of bigger size to be drilled on the circuit board surface for each conductor. With the vias, the total width of circuit board needed is about 30 cm. However, the maximum available width of the circuit board is dictated by the inner diameter of the chamber, which in turn is limited by the minimization of the detector size needed for clinical efficacy and a maximum practical size for maximal imaging signal. For an inner diameter of 11.25 cm (as used for the prototype KCD detector designed for megavoltage imaging), the collector board can be, at the most, 10 cm wide for a 0.5 cm clearance all around the edge required for mechanical alignment.
To be able to route all the channels out of the chamber, one approach is to fabricate the collector board in two parts with half the number of channels on each. The total number of channels will still have to be reduced to 480 (which may be allowed) because of limited width available. Each board can be double-sided with the collecting electrodes on one surface and its routing path via the second surface (back surface). The two boards must be accurately aligned inside the chamber since even a small misalignment can cause a loss in resolution and/or damage to the circuit board and the grid.
Apart from alignment issues relating the two board parts, the collector board itself must be precisely positioned within the chamber, very close to the grid (typically 0.2 B 0.5 mm). Inserting the boards from both ends introduces a blindfolded step in the assembly process; since at least one board must be inserted through the end cap after the detector is assembled, making it difficult to determine when the two boards exactly meet. If the boards are pressed too hard against each other, they might damage each other and cause bowing of the board, ultimately short-circuiting the grid. On the other hand, if the boards are not pressed enough, they may not be touching each other at all, thereby causing a loss of signal and vital channel information. To ensure uniform signal collection, the two boards have to be on the same level and exactly touching, so that the spacing between the first conductors on the two boards is the same as the spacing between all other conductors. This spacing requires tight tolerances and a highly complicated design to get a working chamber, making this solution impractical.
Another approach to the above problem is to reduce the number of channels so that they can be accommodated in the available circuit board width i.e. 9 cm. This means fewer channels to collect the signal, or in other words, a lower spatial resolution or a reduced field of view, either of which is unacceptable. Yet another solution is to increase the tube diameter, which is not practical due to space and weight limitations imposed by the clinical gantry.
Multilayering technique in circuit boards is the most effective conventional approach to address the above problem. Conventional multilayer printed circuit boards consist of a number of thin boards sandwiched together between layers of epoxy resin-impregnated glass cloth. Connections between layers are achieved by means of electroplated holes or vias. The 576 channels on the surface of the circuit board can be routed on different layers that are bonded together. By routing the channels on to different layers, the width of the board no longer dictates the number of channels. Using this technique, the channels on the circuit board can be made to exit from only from one end of the chamber, rather than two ends. This way, there are no complicated alignment issues since there is only one circuit board and the blindfolded step is eliminated.
The manufacturing process for multilayer printed circuit boards starts with the fabrication of the individual component layer boards, excluding the outermost layers. Each of the internal layers needs to be fabricated from thinner substrates than for stand-alone boards. Each internal layer needs to be thoroughly cleaned in abrasive slurry to remove any contaminants. The layers are then baked in an oven to drive off any gases, which could cause separation of the final board due to bubble formation during bonding. The layers are built up as a sandwich on a jig that achieves the necessary alignment using jigging holes. The various internal layers are added, each layer being interleaved with a layer of epoxy-resin-impregnated glass cloth. A sheet of copper foil and a protective film complete the sandwich structure and form the second-outermost layer. It is vitally important that the jigging ensures the correct alignment of the constituent layers during this process. Once assembled, the board is bonded together by heating in a press. After pressing, a routing process is performed to remove any resin material that has been extruded around the edges of a board during pressing.
The processing commences with the pre-processing and CNC drilling processes. It is important that the drilling process is carefully controlled so as to produce a smooth hole and to minimize the amount of resin smear.
The next process is the through-hole plating. This plating process must connect the internal copper layers of the board in addition to connecting the outer copper layers together. Cleaning and the application of solder are then used to finish the construction of the multilayer board.
Based on the need for staggered conductor connections, the circuit board will have to undergo multiple electroplating processes, making the technique expensive. There are other problems too with this conventional multilayering. Holes, with pads, are introduced on the front surface, that are bigger in size than the conductor width, thereby reducing the space between them and increasing the capacitance. The presence of these holes can also restrict the number of available channels. Apart from this, other problems like high transmissions of acoustic vibrations and rigid connector board geometry are also significant.
Therefore, there exists a need in the art for a more compact circuit board having a much greater ratio of open area to folded area so as to be usable in devices where space is limited or overall a reduction in size is desired, and a multilayer circuit board having a much less complicated manufacturing process.
Accordingly, the present invention provides a heretofore unknown approach to folding a flexible circuit board to achieve maximal compactness. This folding is done by determining the folding pattern as a function of the space available for the circuit board, the density of conductor lines, conductor routing constraints, and the mechanical properties of the flexible circuit board.
To ensure that the folded circuit board pattern is highly reproducible and meets a high degree of alignment needed in certain applications (such as a KCD detector), the folding pattern should involve a repeatable-layered geometric pattern. The actual folding pattern to achieve optimal compactness depends on the application and space available. The folded circuit board can be left loose or laminated depending on the mechanical constraints. As well, multiple rigid and non-rigid fasteners can be used to increase the mechanical integrity and utility of the folded circuit board.
Accordingly, it is an object of the present invention to provide a multilayer folded flexible circuit board comprising:
a lower rigid portion having a circuit pattern arranged thereon;
at least one lower foldable strip having a circuit pattern thereon adapted at one end for connection to the circuit pattern of the lower rigid portion and electrically connected thereto;
at least one intermediate portion having a circuit pattern arranged thereon which at a first end is electrically connected to an opposite end of the at least one lower foldable strip;
at least one upper foldable strip having a circuit pattern arranged thereon which is electrically connected to the at least one intermediate portion at a second end of the intermediate portion; and
a connector board electrically connected to an opposite end of the at least one upper foldable strip, the connector board adapted for receiving connectors to provide external attachment to the folded flexible circuit board; wherein
the at least one lower foldable strip and the at least one upper foldable strip fold along diagonal lines in opposite directions of each other.
According to an aspect of the invention, the diagonal folding lines of the lower and upper foldable strips are arranged at respective 45xc2x0 degree angles across each respective foldable strip.
According to another aspect of the invention, the at least one lower foldable strip comprises of four equally sized foldable strips having space therebetween, and the at least one intermediate portion comprises four rigid portions, and
The four equally sized lower foldable strips and the four equally sized intermediate portions have a matching number of electrical patterns so that a particular lower foldable strip electrically connects to a particular respective intermediate rigid portion of the four equally sized intermediate portions.
According to still another aspect of the invention, the at least one upper foldable strip comprises four equally side foldable strips having space therebetween, each respective upper foldable strip is electrically connected to a respective intermediate portion of the four equally sized intermediate portions.
According to yet another aspect of the invention, the at least one lower foldable strip and the at least one upper foldable strip may be prefolded to impart a specific folding point across the respective lower or upper foldable strip.
According to another aspect of the invention, the at least one lower foldable strip and the at least one upper foldable strip are diagonally folded in opposite directions of each other.
The lower rigid portion may be laminated subsequent to the diagonal folding of the lower and upper foldable strips.
In addition, the lamination of the intermediate portions can include FR4 on its backside.
According to another aspect of the invention, one of either right-most or the left-most lower foldable strip further comprises a tab on an outer side thereof.
The lower foldable strips and the upper foldable strips may be diagonally folded in opposite directions.
The lower foldable strips and the upper foldable strips can be diagonally folded in opposite directions, so that the intermediate portions are at least partially stacked on each other.
The lower rigid portion is stacked onto the stack of rigid intermediate portions by horizontally folding the lower foldable strips so that the lower rigid portion is flipped upward onto the intermediate portions.
According to another aspect of the invention, the connector board is stacked onto the upper foldable strips by horizontally folding the upper foldable strips so that the connector board is flipped onto the upper foldable strips, and the upper foldable strips are at least partially stacked on the intermediate portions.
According to another aspect of the invention, the stack of the intermediate portions, the lower rigid portion, the upper foldable strips and the connector board are all bonded together as a composite multilayer structure.
In a second embodiment of the invention, a Kinestatic Charge Detector comprises the folded flexible circuit board according to the present invention, wherein
the lower rigid portion comprises a collector board; and
the Kinestatic Charge Detector comprises a tubular chamber; and the folded flexible circuit board is arranged within the tubular chamber so that only the connector board extends therefrom.
A process for manufacturing a folded flexible circuit board comprising the steps of:
(a) providing a circuit board comprising a lower rigid portion, a plurality of lower foldable strips electrically connected to the lower rigid portion, a plurality of intermediate portions connected at a first end to a second end of the lower foldable strips, a plurality of upper foldable strips connected to a second end of the intermediate portions, and a connector board connected, to an opposite end of the upper foldable strips;
(b) folding the lower foldable strips and the upper foldable strips diagonally in opposite directions to each other so that each of the intermediate portions are at least partially stacked upon each other;
(c) folding the lower rigid portion back on the stack of intermediate portions by folding the lower foldable strips horizontally so that the lower rigid portion stacks on the intermediate portions; and
(d) folding the connector board onto the upper foldable strips and the stack of intermediate-rigid portions by folding the upper foldable strips horizontally so that the connector board stacks on the upper foldable strips and the intermediate portions.
Another aspect of the method according to the present invention further comprises:
(e) bonding the folded flexible circuit board into a folded composite multilayer structure.
In yet another aspect of the method according to the present invention, step (b) first includes:
(i) providing diagonal prefolds on the lower foldable strips and the upper foldable strips so that the strips fold in opposite directions.
According to another aspect of the method of the present invention, step (a) includes providing a tab to at least an outer edge of one of a right-most and left-most intermediate portion.
Finally, the method according to the present invention may include laminating a backside of the intermediate portions.